1. Field of the Invention
The present invention relates to a cryopump and a method for diagnosing the cryopump.
2. Description of the Related Art
A cryopump is a vacuum pump that captures and pumps gas molecules by condensing or adsorbing molecules on a cryopanel cooled to an extremely low temperature. A cryopanel is generally used to achieve a clean vacuum environment required in a semiconductor circuit manufacturing process.
For example, Patent Document 1 describes a cryopump in which a rotational speed of an expander motor is controlled in order to maintain a temperature or a pressure at a constant value. In the cryopump, when a temperature of a cryopanel is increased by performing sputtering, etc., during its operation, the rotational speed of the expander motor falls outside the acceptable range, even when operating normally. Therefore, the cryopump outputs an alarm signal when the rotational speed of the motor falls outside the acceptable range many times in a row. When the rotational speed of the expander motor reaches the upper limit, or the rotational speed is close to the upper limit although not reaching the upper limit, before the target time T1, the cryopump also outputs an alarm signal because it is diagnosed that the cryopump should be subjected to maintenance.
[Patent Document 1] Japanese Patent Application Laid-Open No. H7-293438
However, in the aforementioned cryopump, when a process such as sputtering is performed in a vacuum chamber to be evacuated, the rotational speed of the expander motor falls outside the acceptable range both of during the normal operation and in failure; hence, failure of the cryopump cannot be detected accurately.
Further, an alarm signal is to be outputted when the rotational speed of the motor falls outside the acceptable range many times in a row, therefore there is a possibility that it may be too late when an alarm signal has been outputted. Namely, there is a fear that the cryopump needs to be repaired or replaced immediately after an alarm signal is outputted. In this case, the process currently performed in the vacuum chamber has to be halted, resulting in a failure to manufacture products as scheduled.
In determination of a maintenance timing with the use of the upper limit of the rotational speed of the expander motor, there is a fear that the maintenance timing may be erroneously determined to come even when the cryopump operates normally, because an actual rotational speed of the motor can temporarily exceed the upper limit even when the cryopump operating normally in certain processes.